Some assumptions of the general theory of cognitive tasks are considered in connection with psychological and educational problems. Solution of a cognitive task demands improvement of a particular knowledge possessed by the solver. This improvement consists in ensuring of sufficient completeness (in terms of both the volume and the adequacy) of the information about the object of cognition which constitutes that particular knowledge. The structure of cognitive tasks is described as well as main ways of solving them. A classification of three-component cognitive tasks is given. Results of psychological experiments and some instructional designs are interpreted in the light of the described assumptions.